The present invention relates to formulations suitable for use as fluids for transmissions, especially continuously variable transmissions.
Continuously variable transmissions (CVT) represent a radical departure from conventional automatic transmissions. The push belt version of the CVT was invented by Dr. Hub Van Doorne, and since its introduction, many cars have been equipped with the push belt CVT system. CVT push belts are manufactured by Van Doorne's Transmissie VB of Tilburg, the Netherlands. A more detailed description of such transmissions and belts and lubricants employed therein is found in European Patent Application 753 564, published Jan. 15, 1997, as well as references cited therein. In brief, a belt and pulley system is central to the operation of this type of transmission. The pulley system comprises a pair of pulleys with a V-shaped cross-section, each consisting of a moveable sheave, a fixed sheave, and a hydraulic cylinder. Between the pulleys runs a belt, which consists of a set of metal elements held together by metal bands. In operation, the driving pulley pushes the belt to the driven pulley, thereby transferring power from the input to the output. The transmission drive ratio is controlled by opening or closing the moveable sheaves so that the belt rides lower or higher on the pulley faces. This manner of operation permits continuous adjustment of gear ratio between the input and output shafts.
It has become clear from commercial use of the CVT that the fluids used in the CVT are just as important as the mechanical design for satisfactory operation. The lubricant must fulfill several functions: to lubricate the metal belt in its contacts with the pulley assembly, the planetary and other gears, the wet-plate clutches, and the bearings; to cool the transmission; and to carry hydraulic signals and power. The hydraulic pressure controls the belt traction, transmission ratio, and clutch engagement. The lubricant must provide the appropriate degree of friction between the belt and pulley assembly, to avoid the problem of slippage on one hand, and binding on the other, all the while providing protection to the metal surfaces from pitting, scuffing, scratching, flaking, polishing, and other forms of wear. Accordingly, the fluid should maintain a relatively high coefficient of friction for metal/metal contact, as well as exhibiting a suitable degree of shear stability.
PCT Patent Application WO 00/70001, November, 2000, discloses formulations suitable for use as fluids for continuously variable transmissions, comprising (a) an oil of lubricating viscosity; and (b) a dispersant; or (c) a detergent; or mixtures of (b) and (c), wherein at least one of the dispersant (b) and the detergent (c) is a borated species and wherein the amount of boron supplied to the formulation is sufficient to impart improved friction and anti-seizure properties to said formulation.
U.S. Pat. No. 5,759,965, Sumiejski, Jun. 2, 1998, discloses antiwear enhancing composition for lubricants and functional fluids. It includes a boron-containing overbased material, a phosphorus acid, ester, or derivative, and a borated epoxide or borated fatty acid ester, and optionally a thiocarbamate.
The metal-metal coefficient of friction and the antiseizure properties of CVT fluids are important performance parameters for the effective application of continuously variable transmissions. The present invention solves the problem of providing a suitable CVT fluid with exceptional metal-metal friction and good antiseizure properties. The formulations exhibit both a high dynamic coefficient of friction (metal on metal) and a positive slope in the plot of dynamic friction versus sliding speed.
The compositions of the present invention can also be used as lubricating oils and greases useful in other industrial applications and in automotive engines, transmissions and axles. These compositions are effective in a variety of applications including crankcase lubricating oils for spark-ignited and compression-ignited internal combustion engines, including automobile and truck engines, two-cycle engines, aviation piston engines, marine and low-load diesel engines, and the like. They are also useful as additives for traction fluids. Also, automatic transmission fluids, manual transmission fluids, transaxle lubricants, gear lubricants, metalworking lubricants, hydraulic fluids, and other lubricating oil and grease compositions can benefit from the incorporation of the compositions of this invention. The inventive functional fluids are particularly effective as automatic transmission fluids, particularly fluids for continuously variable transmissions, including push-belt type and toroidal traction drive transmissions, as well as dual clutch transmissions.